The invention concerns glass-ceramic bent plates, cooking plates including such a bent plate and a manufacturing procedure for a glass-ceramic bent plate.
Sales of glass-ceramic cooking plates have been growing continually for several years. They essentially consist of a flat glass-ceramic plate fastened to a case containing the electrical equipment, i.e., the heating elements, the regulation system and several electronic control and display devices. The adjustment knobs of the heating elements are often grouped in a control area which may be provided on the glass-ceramic plate itself or on a strip adjacent to the flat glass-ceramic plate.
The tightness and the mechanical connection between the glass-ceramic plate and the case is currently achieved with a joint made of a polymer, preferably silicone. The glass-ceramic plates are generally decorated with enamels. In addition to the esthetic effect sought, the enamels make it possible to indicate the hot areas.
Different heating systems exist and may be associated with the same cooking plate: classic radiating burners consisting of heating resistors, halogen burners consisting of halogen lamps, mixed radiating burners consisting of both preceding elements, gas burners, induction burners consisting of an inductive loop.
Glass-ceramic is made from glass, called a precursor glass, the specific chemical composition of which makes it possible to cause, by means of adapted heat treatment, called ceramming, controlled crystallization. Their partially crystallized structure bestows unique properties on the glass-ceramic. In the course of the ceramming treatments, the following phases are generally observed: a nucleation phase in the course of which the cores on which the crystals will form coalesce, and a crystallization phase in the course of which the crystals form and then grow. The viscosity of the glass diminishes during the initial heating, presents a minimum just before crystallization and then increases under the effect of the crystallization. The nucleation occurs around 700.degree. C. The crystallization in transparent material occurs around 900.degree. C., while that of the opaque material occurs around 1100.degree. C., with opacification starting around 1000.degree. C.
The glass-ceramic plates for cooking plates are usually made up of a lithium aluminum silicate glass-ceramic. This type of glass-ceramic has a very low coefficient of thermal expansion, which allows the plates to resist thermal shocks of several hundred degrees. Among the lithium aluminum silicate glass-ceramics, two main varieties are distinguished: the beta quartz glass-ceramics which may be colored black and which, therefore, may be weakly transparent in the visible range and more or less transparent in the infrared range, and the beta spodumene glass-ceramics which have a whitish appearance, are opaque in the visible range and weakly transparent in the infrared range. Because of their optical properties, the use of beta spodumene glass-ceramics is primarily associated with induction heating. Other colors or optical appearances may be attained from glass-ceramics of different compositions or that have undergone different treatments.
All of the currently marketed glass-ceramic cooking plates include a flat glass-ceramic plate.
For different technical or esthetic reasons, it would be desirable to be able to manufacture glass-ceramic cooking plates including a glass-ceramic plate bent at a slight radius of curvature, i.e., one in which the bend presents a radius of curvature of less than 5 cm and in particular less than 2 cm. The availability of such bent plates would make it possible to remedy certain disadvantages of the current cooking plates, which are linked to the fact that only flat glass-ceramic plates are available.
Thus, in the event that liquid spills over the edge of a cooking utensil, this liquid may spread toward the surface of the plate and on the control area, reach the edges of the plate and finally spill over.
Currently, to prevent the control areas from becoming fouled, certain ones are separated from the cooking areas by a vertical plastic element which has the double disadvantage of resisting heat poorly and, in addition, of being hard to clean while the ease with which the glass-ceramic cooking plates are cleaned is one of their assets.
Likewise, certain glass-ceramic cooking plates have a vertical control strip placed either before or below the level of the glass-ceramic plate, either in back of or over the level of the glass-ceramic plate. This strip is generally made of smoked glass or painted black in back, but does not have an appearance similar to that of the black glass-ceramic, which is to the detriment of the esthetics of the cooking plate. In addition, joints made of a polymer are arranged to ensure the tightness between the glass strip and the glass-ceramic plate. These joints are hard to clean and are areas where wastes collect. Furthermore, they are subject to deterioration because of burns.
Finally, the joint material, when it is applied, often has a tendency to ooze out over the glass-ceramic plate and, despite the cleaning done at the time of assembly, traces may persist on the finished product, which damages the appearance of the cooking plate.
The availability of bent glass-ceramic plates would therefore make it possible to have greater freedom in designing glass-ceramic cooking plates and would make it possible to prevent the disadvantages of the plates currently on the market.
The problem is, however, that it is not known how to bend a glass-ceramic plate with a slight radius of curvature without degrading the plate. Transparent and clear beta quartz glass-ceramic convex plates are well known, but these plates produced by natural subsidence of the precursor glass plate when it undergoes its ceramming heat treatment, present a great radius of curvature, in excess of 10 cm and frequently of the order of one meter. These convex plates are designed to be used as panes in heating devices such as fireplaces, inserts and furnaces and are not suitable for use in cooking plates, inasmuch as their large radius of curvature is a source of loss of room or is excessively bulky. Furthermore, these plates do not present the high level of quality required for cooking plates.
The most common techniques used to produce cambered plates is not suitable for the production of articles having a bend with a slight radius of curvature, inasmuch as the minimum viscosity of the precursor glass reached at the time of the thermal ceramming treatment is too high to allow the formation of such a bend. Furthermore, if an attempt is made to heat locally a plate made of glass-ceramic precursor glass or of glass-ceramic, as is done for a glass plate, in order to bend it, the visual appearance of the plate is strongly degraded at the heated site because of the development of anarchical crystallization, the heterogeneity of which is also a source of tension and fissuring in the plate.
Accordingly, there continues to be a need for new practical and effective methods of forming glass-ceramic articles, having bends of low radii of curvature.